Radiation Protection in Radiotherapy

Radiation Protection inRadiotherapyPart 2Radiation PhysicsIAEA Training Material on Radiation Protection in Radiotherapy

Part 2, lecture 1: General radiation physics

Radiation Protection in Radiotherapy

BackgroundRadiation generation, transport and interaction with matter are physical processes:While radiation cannot be seen or felt, it can be well described and physically quantifiedIt can be accurately determined using appropriate experimental set-ups



Objectives of the ModuleTo be familiar with different types of ionizing radiationTo understand the most important interaction processes between radiation and matterTo be able to use and understand all basic radiation quantitiesTo have a basic understanding of the means of radiation detection



ContentsLecture 1: GeneralRadioactivityTypes of ionizing radiationInteraction of radiation with matterRadiation quantities and unitsLecture 2: EquipmentBasic means of radiation detection


Radiation Protection inRadiotherapyPart 2Radiation PhysicsLecture 1: General Radiation PhysicsIAEA Training Material on Radiation Protection in Radiotherapy



Radiation = Ionizing RadiationSufficient energy to ionize atoms (eject an electron or add an additional one)This leaves a charged ion.The ion will upset chemical bondsIf this affects critical molecules such as DNA (either directly or indirectly) this can result in cell damage, mutation or death.



Contents1. Radioactivity2. Types of ionizing radiation3. Interaction of radiation with matter4. Radiation quantities and units



Identification of an IsotopeNucleusAtomElectrons



Henri Becquerel (1852-1908)Discovered radioactivity in 1896



1. RadioactivityA property of nucleiDue to inherent physical properties, a nucleus may be not stable and likely to undergo a nuclear transformation. This process can be fast (short half life) or slow (long half life). In any case, the time of transformation cannot be predicted for an individual nucleus - it is a random event which can only be adequately described using statistics



Half life t1/2Describes how fast a particular nucleus transformsThe time it takes for half the amount of a radioactive material to transform (often also referred to as decay)



A(t) = A(0) exp(-t ln2 / t1/2)A(t) activity at time tA(0) original activity at time 0t timet1/2 half life



Half life - logarithmic plot



Types of radioactivityAlpha particles (Helium nuclei) - heavy, dual positive charge, strongly interacting with matterBeta particles/radiation (electrons) - light particle, loosely interacting, still finite rangeGamma radiation (photons)



Alpha decay



Beta decay



Gamma transitionExcited state



RadioactivityMore information on radioactive isotopes used in radiotherapy are provided in part 6 of the present courseMore information on radioactivity is also provided in the companion course on nuclear medicine - in radiotherapy typically the radiation itself is the main consideration...



2. Ionizing RadiationRadioactivity is ONE source of ionizing radiationDeposits an amount of energy in matter which is sufficient to cause the breaking of chemical bondsWave and particle descriptions are both used and correct representationsOne radiation particle often deposits energy at multiple sites - either directly or via the creation of other particles



Types of Radiation (1)X Rays and gamma rays = photonselectrons and beta particles - negative chargeneutronsprotons - positive chargealpha particles and heavy charged particles



Types of Radiation (2)X Rays and gamma rays = photonselectrons and beta particles - negative chargeneutronsprotons - positive chargealpha particles and heavy charged particlesPhotons and electrons arethe most important types ofradiation in Radiotherapy



PhotonsGamma-rays: monoenergetic (one or more lines)X Rays: a spectrumThe difference lies in the way of production:Gamma in nucleusX Ray in atomic shellCW Roentgen, discoverer of X-rays



X Ray productionHigh energy electrons hit a (metallic) target where part of their energy is converted into radiationtargetelectronsX RaysLow tomediumenergy(10-400keV)High > 1MeVenergy



Issues with X Ray productionAngular distribution: high energy X Rays are mainly forward directed, while low energy X Rays are primarily emitted perpendicular to the incident electron beam - this is reflected in the target designtargetLow tomediumenergy(10-400keV)High > 1MeVenergy



X Ray Tube for low and medium X Ray production



Megavoltage X Ray linac



Issues with X Ray productionAngular distribution: high energy X Rays are mainly forward directed, while low energy X Rays are primarily emitted perpendicular to the incident electron beamEfficiency of production: In general, the higher the energy, the more efficient the X Ray production - this means that at low energies most of the energy of the electron (>98%) is converted into heat - target cooling is essential



Types of X Ray production Characteristic X Rays: 1 The incoming electron knocks out an inner shell atomic electron 2 An electron from a higher shell fills the vacancy and the energy difference is emitted as an X Ray of an energy characteristic for the transition12



Types of X Ray productionBremsstrahlung: The incoming electron is deflected in the atomic shell and decelerated. The energy difference is emitted as an X Ray



Bremsstrahlung productionThe higher the atomic number of the X Ray target, the higher the yieldThe higher the incident electron energy, the higher the probability of X Ray productionAt any electron energy, the probability of generating X Rays decreases with increasing X Ray energy



The resulting X Ray spectrumCharacteristicX RaysBremsstrahlungSpectrum afterfiltrationMaximum electron energy



The effect of additional filtration



Types of Radiation (3)Directly ionizing radiation - energy is deposited by the particle directly in matter (electrons, protons)Indirectly ionizing radiation - primary particle transfers energy to secondary particle which in turn causes ionization events (photons, neutrons)



3. Interaction of radiation with matterDetermines penetration (how much radiation reaches a target)Determines dose deposited in the target?



Types of RadiationIonizationEvents



Which one is indirectly ionizing ?IonizationEvents



Comparison of depth dose characteristics



photons are most commonly used



Photons are part of the electromagnetic spectrum



Photons are part of the electromagnetic spectrumEnough energyto cause ionization



Photon Interactions



Albert EinsteinExplanation of the photo-effect





Variation of photon interaction coefficient with energyTherapeuticX Ray range



Variation of attenuation with atomic number



Variation of attenuation with atomic number



ConsequencesLead shielding very efficient at low photon energies (diagnostics)In general, photons are difficult to attenuate, in particular in the megavoltage range used for therapyMegavoltage photons are less suitable for imaging



Secondary and tertiary particles in a megavoltage photon beam



Electron interaction in matterIonization events and excitation of atoms all along the electron path in matter. Individual energy depositions are small and a megavoltage electron may deposit energy at >10000 locationsBremsstrahlung (= braking radiation). The electron loses energy in form of X Rays as it is deflecting around nuclei



BremsstrahlungMost effective for electrons of very high energy in materials of high atomic number (metals).The production process of X Rays in the first placetargetX Rayselectrons



Electron interactions



Electron interactionsTertiary photonsfrom Bremsstrahlung



Photons ElectronsExponential attenuationIndirectly ionizingFinite rangeDirectly ionizing



From radiation to energy deposition in a photon beam



4. Radiation quantities and units Need to quantify radiation effects todetermine and quantify risks determine the likelihood of benefit (cancer cure or palliation)to weigh risk and benefit to optimize radiotherapy approachesto make informed decisions



Characterization of radiationSourceEnergyDepositionFirstInteractionTransport



Physical quantities which can be measuredAt the source: Activity, mA, kVOn the flight: Flux, fluenceAt the first interaction point: Kinetic Energy Released in Matter (KERMA)In matter: Absorbed dose



ActivityThe amount of a radionuclideSI unit is the Becquerel (Bq) - one nuclear transformation per secondOld unit is the Curie (Ci)1 Ci = 37 x 109 Bq = 37 GBq



1 Bq is a small quantity40-Potassium in every person > 1000BqMany radioactive sources are > 100,000BqRadioactive sources in radiotherapy often > 100,000,000Bq



Multiple & prefixes (Activity)Multiple Prefix Abbreviation1 -Bq1,000,000Mega (M)MBq1,000,000,000Giga (G)GBq1,000,000,000,000Tera (T)TBq



ExposureNumber of charges created by radiation in airRelatively easy to determine Measured in coulomb per kilogram (C/kg) - old unit roentgen (R) 1 R = 2.58 x 10-4 C/kg



Absorbed DoseEnergy deposited in matterD = E/m (1 Gy = 1 J/kg)The unit related to effects in matterNot necessarily a straight forward relationship to the intensity of the radiation beam



From Exposure to DoseEXPOSUREonly defined in airfirst impact quantityDOSEcan be defined in any medium using stopping power ratioscan be derived from exposure using W/e



1 Gy is a relatively large quantityRadiotherapy doses > 1GyDose from diagnostic radiology typically < 0.001GyAnnual background radiation due to natural radiation (terrestrial, cosmic, due to internal radioactivity, Radon,) about 0.002Gy



Fractions & Prefixes (Dose)Fraction Prefix Abbreviation

1 -Gy1/1000milli (m)mGy1/1,000,000micro ()Gy



SummaryIn radiotherapy, photons (X Rays and gamma rays) and electrons are the most important radiation typesThere are several different interaction processes possible for photons - all important ones transfer energy to an electron which deposits the energy in tissue.Absorbed dose is defined as energy deposited in matter and measured in Gray



Where to Get More InformationMedical physicistsTextbooks:Khan F. The physics of radiation therapy. 1994.Metcalfe P.; Kron T.; Hoban P. The physics of radiotherapy X-rays from linear accelerators. 1997. Cember H. Introduction to health physics. 1983Williams J; Thwaites D. Radiotherapy Physics. 1993.



A note of caution:Energy deposition inmatter is a randomevent and thedefinition of dosebreaks down forsmall volumes (e.g. a single cell). Thediscipline of Micro-dosimetry aims toaddress this issue.Adapted from Zaider 2000


Any questions?


Question:What difference would one expect when using megavoltage photons for imaging of patients instead of the kilovoltage X Rays used in diagnostic radiology?



Simulator (kV) and portal image (MV) of the same anatomical site (prostate)Reference simulator filmCheck portal film



AcknowledgmentRobin Hill, Liverpool Hospital, Sydney


Part No 2, Lesson No 1Part 2: Radiation PhysicsModule 1: Two lectures as explained in slide 4Lesson 1: Basic radiation physicsLearning objectives: To become familiar with different types of ionizing radiationTo understand the most important interaction processes between radiation and matterTo be able to use and understand all basic radiation quantitiesTo have a basic understanding of the means of radiation detectionActivity: lecture - practical exercisesDuration: lecture 1: 2; lecture 2: 2Materials and equipment needed: Medical Physics textbookReferences: Johns H E; Cunningham J R. The physics of radiology. Springfield: CC Thomas; 1983. Khan F. The physics of radiation therapy. 2nd edition. Baltimore: Williams & Wilkins; 1994.Metcalfe P.; Kron T.; Hoban P. The physics of radiotherapy X-rays from linear accelerators. Madison: Medical Physics Publishing; 1997.VanDyk, J. Modern Technology of Radiation Oncology (Ed.: J Van Dyk) Medical Physics Publishing, Wisconsin 1999, ISBN 0-944838-38-3 Williams J; Thwaites D. Radiotherapy Physics. Oxford: Oxford University Press; 1993.

IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Point out the importance of physics as background to understanding radiationIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Explanation or/and additional informationThe aim of this module is to have the foundation to understand the most important factors in radiotherapy1. Successful patient treatment2. Safe application including radiation safety

IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Explanation or/and additional informationThere is too much material to cover in one lecture. The first lecture can be shortened for professionals with a background in radiation physics - eg physicists with experience in diagnostic imaging

Instructions for the lecturer/trainer

IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Part 2: Radiation PhysicsModule 1: Lecture 1Lesson 1: (Add session number and title)Learning objectives: Duration: 2 hours

IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Point out that radiation is a general term for many physical (and non-physical) phenomena. The course here is only concerned with ionising radiation.References to non-ionising radiation (ICNIRP) could be given.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Explanation or/and additional information

Instructions for the lecturer/trainer

IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The lecture assumes familiarity with basic physics concepts - the present slide allows the lecturer to review the basic atomic model and the nomenclature for isotope labeling.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Depending on the background of the participants, the equation on the following slide could be left out.However, in general one would expect that all participants are familiar with the basic shape of an exponential curve. The lecturer should point out:similarity with photon attenuationactivity NEVER gets down to 0

IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1This plot can be used by the lecturer to re-introduce the participants to the logarithmic scale.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The isotopes involved here are transuranium - they are Segborium and RutherfordiumIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The lecturer can point out that the neutrino which is also part of the decay equation is needed for conservation of energy and momentum. In the context of the present course it is of no consequence since its probability of interaction with matter is very small.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The isotope shown here is DysprosiumThe lecturer can point out that not decay is mentioned but transition - this is due to the fact that the isotope remains from the same element. Just some internal energy is lost and emitted in the form of electromagnetic radiation.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1This slide leads from section 1 of the lecture to section 2...IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Explanation or/and additional informationThe next slide states the important fact that electrons and photons are the most important radiation types for radiotherapy.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Explanation or/and additional informationThe lecturer can state that in the following we will learn more about the two types of radiation of interest.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The lecturer could point out that the difference between the two types of radiation does NOT reflect on energy (it is sometimes stated that gamma rays are of higher energy than X-rays - this is not correct)IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The intention of this slide is not to cover X-ray production extensively. The important issues for the lecturer to point out are mentioned on the next slide.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1In both this and the next slide it is not important to go through all the details of the X-ray production - the important features to note are the target design and the angle of incidence.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The lecturer can mention that at high energies the efficiency of X-ray production can exceed 50% - however, target cooling is still a major issue because of the high beam currents.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The contents of this slide should follow from the previous discussions. The important points which should be mentioned by the lecturer are given in boxes. An alternative teaching approach would be to delete one or more of the boxes and ask the participants what is displayed.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Unlike in diagnostic radiology, this slide is used here just to illustrate the fact that low energy X-rays are preferentially absorbed by additional filtration.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Explanation or/and additional information

IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The lecturer should only describe the different processes:Photon - attenuation exponentially in matter - interaction with electrons resulting in a secondary electron depositing energy in matterElectron - interaction with other electrons with usually small transfer of energy. Therefore continuous slowing down. After all energy has been transferred to matter the electron is locally absorbed. This leads to a finite range of electrons in matter.Protons - similar to electrons, however more densely ionizing. More interactions per unit length. Higher energy required for same range in matter as electrons. As a heavier particle also less deviations from original path. Therefore, less angular struggling, less lateral spread of the beam (better penumbra) and better defined range.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The question is aimed at stimulating the students and providing feed back to the lecturer. The photon is the indirectly ionizing particle - it transfers parts of its energy onto the secondary electron which then distributes the energy in matter. If time and background of the students permits, the lecturer can point out that also neutrons are indirectly ionizing particles transferring energy to eg. Protons.

IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1This slide can be used to illustrate two things:1. Indirectly ionizing radiation is attenuated exponentially - the same would be true in eg. Shielding material. Therefore a photon beam will never be completely absorbed. Directly ionizing radiation has a finite range depending on the energy of the incident particle.2. The slide allows a deviation into the radiotherapy context - typical patient dimensions are of the order of 20cm (head) and 30-40cm (torso). One can see what percentage of dose is deposited by different radiation types at this depth.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The lecturer can point out that the wide spread use of photons is not necessarily linked to their favourable depth dose characteristics - protons would be more desirable. However, photons are the easiest and cheapest penetrating radiation type available.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1This slide rounds up the discussion of ionising vs non-ionising radiationIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Lecture notes: ( about 100 words)

Instructions for the lecturer/trainerThe lecturer can point out that all of these interactions result in energy transfer onto an electron which then in turn deposits the energy in matter (indirectly ionizing radiation)IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1More details on the interaction types - this slide could be used as a handout.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The following three slides illustrate the variation of interaction coefficients with photon energy. The lecturer can use this slide to review again the different interaction coefficients and their energy dependence. The area of interest in the therapeutic X Ray range is pointed out, where the lecturer can show the interactions of main impact in this energy-range.Please point out the double logarithmic scale

IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The lecturer can use this slide to explain the fate of a photon and the different possibilities how secondary electrons can interact with matter. The important point to highlight is that only electrons (= directly ionizing radiation) deposit significant amounts of energy in matter.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1This slide can be used to recap the X-ray production mentioned earlierIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Monte Carlo calculation (EGS4) of the dose deposition of a 12MeV electron beam. The lecturer could ask what the straight lines are. The answer is given on the next slide.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1This will be discussed in more depth in part 10 of the courseIAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Attenuation is the most important property for medical imaging. Differences in the number of photons behind an inhomogeneous absorber create contrast.

In the second step illustrated in the slide we consider only the interactions which actually transfer energy to the matter. This means we exclude all elastic scattering processes such as Thompson scattering. This yields the Total Energy Released in Matter or TERMA. However one is also not interested in the scattered photons, which leave the object after some energy loss (eg. after Compton interaction). If we exclude also this component we obtain the energy which is actually transferred from the photon to electrons or - less importantly - the nuclei. This is also termed Kinetic Energy Released in Matter or KERMA and the coefficient describing this is called mass transfer coefficient m tr/r .IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1This slide introduces the notion of how radiation can be quantified - there are different steps where this can occur. This is discussed in the following in more detail.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1This is an insertion to help make sure all participants are at the same level.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1Lets summarize the main subjects we did cover in this session.

IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1The lecturer should point out that the x-axis shows d for dimension or size. Similar figures have been drawn for mass as ordinate. In any case it is evident that the specific energy deposited in increasingly smaller volumes becomes less well defined. At small dimensions the line which defines absorbed dose macroscopically becomes blurred and a particular volume may have received a number of different energy quantities. At very small dimensions and low doses the question comes to either having received an event (or more than one) or not.IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in RadiotherapyPart No 2, Lesson No 1IAEA Training Material: Radiation Protection in Radiotherapy

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Radiation Protection in Radiotherapy